Re: [RFC PATCH v7 5/5] mmc: queue: Use bigger segments if DMA MAP layer can merge the segments

[Christoph Hellwig <hch@lst.de>]

On Thu, Jun 20, 2019 at 05:50:10PM +0900, Yoshihiro Shimoda wrote:
> When the max_segs of a mmc host is smaller than 512, the mmc
> subsystem tries to use 512 segments if DMA MAP layer can merge
> the segments, and then the mmc subsystem exposes such information
> to the block layer by using blk_queue_can_use_dma_map_merging().
> 
> Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
> ---
>  drivers/mmc/core/queue.c | 35 ++++++++++++++++++++++++++++++++---
>  include/linux/mmc/host.h |  1 +
>  2 files changed, 33 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/mmc/core/queue.c b/drivers/mmc/core/queue.c
> index 92900a0..ab0ecc6 100644
> --- a/drivers/mmc/core/queue.c
> +++ b/drivers/mmc/core/queue.c
> @@ -24,6 +24,8 @@
>  #include "card.h"
>  #include "host.h"
>  
> +#define MMC_DMA_MAP_MERGE_SEGMENTS	512
> +
>  static inline bool mmc_cqe_dcmd_busy(struct mmc_queue *mq)
>  {
>  	/* Allow only 1 DCMD at a time */
> @@ -196,6 +198,12 @@ static void mmc_queue_setup_discard(struct request_queue *q,
>  		blk_queue_flag_set(QUEUE_FLAG_SECERASE, q);
>  }
>  
> +static unsigned int mmc_get_max_segments(struct mmc_host *host)
> +{
> +	return host->can_dma_map_merge ? MMC_DMA_MAP_MERGE_SEGMENTS :
> +					 host->max_segs;

I personally don't like superflous use of ? : if an if would be more
obvious:

	if (host->can_dma_map_merge)
		return MMC_DMA_MAP_MERGE_SEGMENTS;
	return host->max_segs;

but that is really just a nitpick and for the mmc maintainer to decide.

Otherwise looks good:

Reviewed-by: Christoph Hellwig <hch@lst.de>